Funds are requested to upgrade an existing commercial Bruker Elexsys 500 continuous wave EPR spectrometer to add pulse capability. The upgraded instrument will be a highly versatile EPR instrument that will be capable of a broad range of pulse experiments with ns time resolution, but will retain the ability to run in CW mode. This instrument is also capable of saturation recovery as well as ELDOR experiments. This will be a multiuser instrument that will be housed and maintained by the Department of Chemistry. The primary user group will consist of five research groups at the University of Virginia: Profs. Bryant, Cafiso and Columbus in the Department of Chemistry, and Profs. Tamm and Nakamoto in the Department of Molecular Physiology. The facility will also be available to other research groups at the University. The primary use of the instrument will be for the determination of intermolecular distances within macromolecules using techniques such as double electron-electron resonance (DEER). A focus of the user group is to obtain information on the structure and dynamics of membrane proteins, and large protein complexes. This information will be used to determine the structures and structural changes in proteins that facilitate transport across biological membranes, the structures and mechanisms of proteins that facilitate membrane attachment in cell-signaling systems, and the mechanisms of protein- mediated membrane fusion. Membrane proteins are an extremely important class of proteins that are targets of the majority of the pharmaceuticals currently in use. Unfortunately, it is quite challenging to understand their molecular function, because they are not easily approached using standard structural methods such as crystallography and high-resolution NMR. As a result approaches using EPR, such as site-directed spin labeling, have become important tools to examine this class of proteins.